Phosphorylation of turkey gizzard and pig stomach myosin is catalyzed by a Ca 2 ion dependent kinase. The kinase from turkey gizzards has been purified to near homogeneity and found to have a specific activity of 10-15 micron moles of P transferred to myosin light chain per minute. It is composed of a polypeptide chain of 130,000-135,000 daltons and is entirely inactive in the absence of a 16,500 dalton polypeptide, Ca 2 ion-dependent regulatory protein (CDR). The phosphatase which dephosphorylates smooth muscle myosin is not dependent on CDR for its activity. Phosphorylation of smooth muscle myosin is required for actin-activation of the myosin ATPase activity. Studies with pig stomach myosin are being carried out to see if Ca 2 ion regulation may also exist at the level of actin-myosin interaction, as well as at the level of the kinase.